1. Field of the Invention
The present invention relates to an image-forming device and an image-forming method. Specifically, the present invention relates to a printing device capable of printing on both sides of a sheet or on multiple sheets, and in particular relates to an image-forming device and image-forming method having a print control method for circulating multiple sheets of cut-sheet paper within the device and performing printing on both sides of the sheet or on multiple sheets.
2. Description of the Related Art
Conventionally, with an image-forming device having a double-sided print function, a method is used to increase printing speed with multiple pages whereby an image printed on a first side of a sheet and an image printed on a second side of the sheet are alternately printed. For example, there may be situations wherein a condition arises the printing cannot be continued because there is no more paper in the paper supply unit during a continuous job. In such a situation, a method has been proposed whereby a job is completed without leaving any paper with uncompleted printing within the device by switching the sheet supply unit specification from the paper supply unit to the double-sided re-supply unit. (e.g., see Japanese Patent Laid-Open No. 01-011272.) The method disclosed in Japanese Patent Laid-Open No. 01-011272 is effective for an image-forming device in a state wherein the existence of paper whereupon the image is formed can be confirmed prior to starting the image-forming.
However, recently, with an image-forming device such as a full-color electronic photography device having an intermediate transfer unit, the distance from the portion performing the first image-forming process to the secondary transfer position whereby the image is transferred from the intermediate transfer unit onto the paper has increased. Therefore, in order to increase the print speed during continuous printing (hereafter also called “throughput”), the supply operation of the paper whereupon the image is to be transferred is performed in advance of starting image-forming. Thus, the portion performing the first image-forming process equates to the portion performing exposure to the image carrying member with an electronic photography device.
FIG. 15 is a configuration diagram illustrating the situation of a supply sheet misprint of an image-forming device having an intermediate transfer unit, and is a general diagram of an image-forming device having a method of forming a primary image by stacking four colors serving as basic colors on the intermediate transfer unit and performing secondary transfer thereof onto the paper. The reference numerals in the diagram T1a, T1b, T1c, T1d each indicate a primary transfer unit of each of the basic colors, T2 indicates a secondary transfer unit, and 2 indicates an intermediate transfer belt (hereafter “ITB”) serving as an intermediate transfer unit. With such an image-forming device, images on multiple pages are formed in advance on the ITB 2 in order to increase throughput. Note that the remaining reference symbols and numerals depicted in FIG. 15 are the same as those depicted in FIG. 2, and the detailed description are omitted herein and provided in the description of FIG. 2 below.
Thus, in the event that paper in the paper supply unit, such as the sheet supplying cassette, runs out during continuous printing, the situation may occur whereby images are already formed on the primary transfer units T1a, T1b, T1c, T1d, or on the ITB 2.
FIG. 9 is a configuration diagram illustrating the situation of a misprint due to the absence of paper in the image-forming device. Note that the reference symbols and numerals depicted in FIG. 9 are the same as those depicted in FIG. 2, and the detailed description of the reference numerals not discussed with respect to FIG. 9 are provided in the description of FIG. 2 below.
The reference symbol 13 in the diagram indicates a paper storing cassette serving as a sheet supplying unit, and PS indicates a detecting sensor for detecting whether or not there is paper in the paper storing cassette 13. The paper supplied from the paper storing cassette 13 is transported to a fixer 10 via a secondary transfer unit 7 wherein the image on the ITB 2 is transferred onto the paper. Reference numeral 16 denotes a transporting path switchover flapper which divides the paper having ended fixing between transportation towards a discharge tray (not shown) side and transportation towards a double-sided unit which reverse-transports the paper. When performing printing on both sides of the paper, the printed side of the paper is switched over at the double-sided unit via the flapper 16, and is re-supplied to the secondary transfer unit 7.
Symbols ID1 through ID4 are identifier symbols assigned by the control system for each print job. ID1 through ID4 specify double-sided printing for any jobs. The order of printing processing is executed alternately with the first side of the sheet and the second side of the sheet, so that after printing execution of the first side of the sheet for ID1, the first side of the sheet for ID2, and the first side of the sheet for ID3, then printing is executed on the second side of the sheet for ID1, the first side of the sheet for ID4, the second side of the sheet for ID2, the first side of the sheet for ID5, and so forth.
As shown in FIG. 9, the sheet for ID3 is supplied in the state of printing being completed on the first side of the sheet as to ID1 and ID2, whereby the side of the sheet for ID1 is in the state of awaiting sheet re-supply within the double-sided unit, and the paper for ID2 is in the state of reverse transporting. Then at the point in time when the trailing edge of the paper has passed the PS, the paper storing cassette 13 is detected to be out of paper. At this point in time, the image of the second side of the sheet for ID1 is already formed upon the ITB 2, and further image-forming as to the first side of the sheet for ID4 and the second side of the sheet for ID2 are also started.
After printing the first side of the sheet for ID3, the second side of the sheet for ID1 has printing executed with paper supplied from the double-sided unit, and so the job can be completed by discharging into the discharge tray. However, because there is no paper serving as the object for the image of the first side of the sheet for ID4, transfer onto the paper cannot be performed.
In the case described above, the paper supply opening cannot be simply switched from a cassette with no paper to the double-sided unit and print the second sheets for ID2 and ID3 before printing the job for ID4, as is disclosed in Japanese Patent Laid-Open No. 01-011272.
This is because in the situation illustrated in FIG. 9, there is no paper to transfer the image for the first side of the sheet for ID4, which is already formed on the ITB2, and so the image for the first side of the sheet for ID4 adheres to the secondary transfer unit T2.
In addition to the above described case where there is no paper, there are also situations where transfer to the paper is not possible due to a paper sheet supplying error.
Currently, in the event that an out-of-paper or sheet supplying error occurs so that transfer to the paper is not possible during a double-sided alternate printing operation, image-forming is stopped. The paper of the sheet where printing has been completed on one side is then discharged to the outside of the device, and printing is redone from the sheet whereupon the miss occurred. Accordingly, the paper of the sheet whereupon printing is completed on one side has been wasted.